This invention relates to segmental cutting tools. Such tools comprise a body member and, affixed to the body member, a plurality of segments comprising the abrasive components that perform the cutting. The most common cutting tools to which this invention relates are core bits, diamond saw blades and segmented wheels. The present invention is applicable to all such tools in which the cutting action is performed by segments attached to a body member.
Core bits are used to drill holes in very hard materials such as rock strata or concrete members. The bit comprises a cylindrical steel body member adapted at one end for attachment to a drill and having a plurality of spaced segments located around the annular rim at the opposed end of the cylinder that perform the cutting function.
Segmented wheels and diamond saw blades comprise a metal disc with a plurality of segments fixed to and spaced around the circumference of the disc to provide the cutting means.
The segments comprise abrasive particles dispersed in a metal bond and these segments are most frequently attached by being welded to the body member. The segments usually have a basically rectangular configuration with one long edge being welded to the body member. In a core bit the "rectangular segment" is bowed along its length to allow the long edge to conform to the annular edge of the cylindrical body member to which it is attached. Thus the segments project from the body member by the amount of their width. The thickness of the segments is conventionally the same as, or a little greater than, the thickness of the edge of the body member to which they are attached.
In a segmented wheel or diamond saw the rectangular segments are also bowed but in this case the are bowed in such a way as to conform a long edge of the segments to the curvature of the rim of the disc to which it is to be attached.
The number of segments and their spacing around the edge of the body member to which they are attached can vary somewhat depending on the size of the body member and the application. In general however, for core drill bits having a diameter of from about 50 mm to 500 cm, from about 2 up to several hundred segments may be used. Smaller or larger diameter drill bits may use fewer or more respectively. Segmented wheels can have from about 8 up to several hundred segments depending on the diameter of the wheel. Such wheels are generally from about 10 cm and up in diameter.
The abrasive component can be any one of those commonly used for such applications, the grit being chosen depending on the hardness of the material to be cut. Thus the grain may be aluminum oxide, silicon carbide, tungsten carbide or a superabrasive such as diamond or cubic boron nitride, (CBN). Superabrasives are usually preferred though the superabrasive component may be diluted with less expensive abrasive grits. The abrasive is typically held in a metal bond and the adhesion to the bond may be enhanced by metal coating the grain with a metal such as nickel, before it is incorporated in the segment.
One of the problems with core bits is in ensuring that the outside gauge of the hole drilled remains constant. This is because the edges of the segments tend to wear away more quickly than the central portion of the segments making the cutting operation slower and less efficient. This can lead to widening of the outer gauge of the hole being drilled and, where there are variations in the hardness of the material being drilled, this too can lead to deviations from the desired direction of drilling.
This problem has typically been addressed by forming the segments with a higher concentration of abrasive along both of the sides of the segment making the segment harder along the outer surfaces. These are often called "sandwich segments". The difference in hardness creates a profile on the surface being cut that provides self-centering of the bit. However this solution is only partially effective because the resultant cut rate is often significantly slowed or the tool life is significantly reduced. It has now been found possible to greatly improve the efficiency of cutting in a surprising and unobvious way using the novel tool design that is the subject matter of this invention. Not only does the design provide faster cutting but in some embodiments it appears to provide longer life and/or an efficient self-centering mechanism that ensures the hole drilled remains straight.